Unactivated and niobium activated monoclinic M' yttrium tantalate (YTaO.sub.4) X-ray phosphors are used in x-ray intensifying screens for medical radiographic applications. Examples of these phosphors are given in U.S. Pat. Nos. 5,009,807, 5,112,524, and 4,225,653, which are incorporated herein by reference.
One property associated with x-ray phosphors which can cause serious problems is the presence of delayed fluorescence. This delayed fluorescence, also known as afterglow, lag, or persistence, is the emission of light from the phosphor after x-ray excitation is stopped. The presence of a large afterglow in a phosphor screen will compromise the quality of the radiographic images collected using that screen. Particularly, x-ray intensifier screens used in auto-changers in hospitals for routine x-ray procedures require low or zero-lag phosphor screens because the x-ray intensifier screens in automated changers are used many times over a short period of time. In such applications, a high lag phosphor screen can retain a part of the previous image which interferes with the new x-ray exposure. Yttrium tantalate phosphors commonly exhibit substantial levels of phosphor lag. Because these phosphors are used to prepare intensifier screens for use in automated rapid exposure X-ray devices, the availability of such phosphors with the lowest possible lag has become increasingly important in order to obtain high radiographic image quality.
Other properties associated with x-ray phosphors are their XOF (X-ray Optical Fluorescence) brightness and particle size. The use of a brighter x-ray phosphor in intensifier screens shortens exposure time and decreases x-ray dosage for patients needing medical X-ray imaging procedures. However, XOF brightness is usually accompanied by an increase the particle size of the phosphor grains. Increased particle size in intensifier screens reduces the resolution of the resulting x-ray images. Thus, preparation of an x-ray phosphor with improved XOF brightness without the concomitant increase in particle size is highly desirable.
Strontium is known to increase XOF brightness and decrease afterglow (persistence and lag) in yttrium tantalates when added to the formulation or to the commonly used Li.sub.2 SO.sub.4 or Li.sub.2 SO.sub.4 --LiCl fluxes as SrCO.sub.3, SrCl.sub.2, SrCl.sub.2.6H.sub.2 O, or other Sr.sup.2+ containing species. The use of eutectic Li.sub.2 SO.sub.4 --LiCl flux increases phosphor brightness but also tends to increase particle size and cause damage to crucibles and ovens. Although Sr increases XOF brightness and decreases persistence, the addition of Sr does not lead generally to the production of a zero lag phosphor and the increase in XOF brightness from Sr addition is not continuous. In particular, high levels of Sr form increased amounts of impurity phases which reduce XOF brightness and increase persistence. Thus, it would be beneficial to have a method to reduce persistence and increase XOF brightness without increasing particle size.